Petroleum refining and petrochemical processes frequently involve separating hydrocarbon components that have very similar structure and properties.
For example, propylene-propane splitters typically comprise distillation towers that are used to separate hydrocarbons streams into polymer grade propylene (PGP) stream as a net overhead stream and a stream with propane in a net bottoms. Due to the low relative volatility of propylene and propane, typically a very large tower with 150 to 250 trays is used. Additionally, the tower also typically requires a reflux to feed ratio of 5 to 10 to make the separation. Since the relative volatility of propylene and propane is so low (typically 1.05 to 1.20), the fractionation is energy intensive in order to separate propylene and propane into high purity component streams.
Typically, a heat pump compressor is utilized to condense (or remove energy) in the fractionation column overhead and re-boil (or feed energy) into the column bottoms because the vapor pressure of propylene and propane are similar and the heat removed from the column overhead for condensing can be transferred or pumped to the tower bottoms for re-boiling.
In some designs, such as the design disclosed in U.S. Pat. Pub. No. 2013/0131417, which is assigned to the Assignee of the present invention, and the entirety of which is incorporated herein by reference, an overhead vapor from a propylene-propane splitter column (“PP Splitter”) is sent to the first stage heat pump compressor. The stream being separated in the PP Splitter is typically from an upstream deethanizer. In the first stage heat pump compressor, the overhead vapor of the PP Splitter is compressed to the required pressure, typically between approximately 1,034 to 1,724 kPag (150 to 250 psig), which is the minimum temperature for a heat exchanger to condense vapor on the hot side and re-boil liquid on the cold side of the heat exchanger. The duty required to re-boil the PP Splitter determines the vapor flow rate to the re-boiler/condensers. Since the condensing duty is greater than the re-boiling duty of PP Splitter, there is excess vapor from the first stage discharge that requires condensation. This extra vapor is sent to the second stage of the heat pump compressor, where it can be compressed to a pressure able to be condensed by another heat exchanger at a warmer temperature. Subsequently, this stream is flashed across a valve into a suction drum down to the column overhead pressure to provide Joule-Thomson effect cooling to the column overhead and accumulate propylene liquid product in the suction drum. In such a system described above, when the second stage discharge stream is flashed down to the column overhead pressure, the resulting vapor from this flash is then re-processed in the heat pump first stage and second stages, sequentially. Thus, the first stage of the heat pump compressor, which is the larger capacity stage requiring more utility, needs to process the column overhead vapor along with the vapor from the second stage discharge flash, thereby increasing the overall capacity and utility requirement of the compressor.
Another system for recovering heat from a PP Splitter is disclosed in U.S. Pat. No. 7,981,256, which is also assigned to the Assignee of the present invention, and the entirety of which is incorporated herein by reference. In the design depicted in U.S. Pat. No. 7,981,256, a multi-stage heat compressor system is used to transfer heat from the overhead stream of the PP Splitter to reboilers for the PP Splitter. This application utilizes at least three stages and still requires an external refrigeration system for the upstream deethanizer.
In yet another design is disclosed in U.S. Pat. Pub. No. 2015/0101921, assigned to the Applicant of the present application, and the entirety of which is incorporated herein by referenced, utilizes a single, 2-stage compressor. The system and processes disclosed in U.S. Pat. Pub. No. 2015/0101921 recognize that the refrigeration system needed to condense the deethanizer rectifier is often expensive. However, the heat removed for the PP Splitter overhead condensation is wasted with air or cooling water.
While these designs are presumably effective for their intended purposes, there is a continuing need to develop and provide processes which improve on energy efficiency and heat recovery.